The present invention relates generally to electrical fuses, and more particularly to an electric fuse having an indicator for indicating whether the fuse has become non-conducting, i.e., xe2x80x9cblown.xe2x80x9d
An electric fuse is designed to allow temporary and harmless current to pass therethrough without triggering, i.e., opening the fuse. The fuse is nevertheless operable to open if subjected to sustained overloads or excessive short circuit conditions. In many instances, an xe2x80x9copen,xe2x80x9d i.e., a xe2x80x9cblown,xe2x80x9d fuse will exhibit no visible signs of its condition.
The present invention relates to an improved electric fuse having an indicator for indicating whether the fuse has become non-conducting, i.e., xe2x80x9cblown.xe2x80x9d
In accordance with a preferred embodiment of the present invention, there is provided an electric fuse comprised of a tubular fuse casing formed of an electrically insulating material. A first conductive component is attached to a first end of the casing and a second conductive component is attached to a second end of the casing. A first conductive path is formed through the tube between the first and second conductive components. The conductive path includes a fusible element having a first resistance. A second conductive path is formed along the exterior of the tubular fuse casing. The second conductive path is in parallel to the first conductive path and has a second resistance greater than the first resistance. The second conductive path includes an indicator component. The indicator component is comprised of first layer comprised of a deep-dyed color material, and a second layer comprised of a conductive material deposited onto the first layer. The second layer has a region of increased resistance. An inner cavity is located above the region of increased resistance on the second layer. A third layer comprised of a transparent, polymeric material covers the cavity and the first and second layers.
In accordance with another aspect of the present invention, there is provided an electric fuse, comprised of a tubular fuse casing formed of an electrically insulating material having a first conductive component attached to a first end of the casing and a second conductive component attached to a second end of the casing. A first conductive path is formed through the tube between the first and second conductive components. The first conductive path includes a fusible element and has a first resistance. An indicator strip extends along the length of the tubular fuse casing. The indicator strip is comprised of a first layer of a colored polymer having a layer of metal deposited thereon. The metal layer is electrically connected to the first and second conductive components to be in parallel with the first conductive path. The metal layer has a resistance greater than the first conductive path and a region of reduced cross-sectional area wherein the resistance of the metal layer in the region of the reduced cross-sectional area is greater than a remainder of the metal layer. A cover layer of polymeric material covers the indicator strip. The cover layer is clear in the vicinity of the region of reduced cross-sectional area, wherein the region is visible through the cover layer. A cavity is formed between the cover layer and the metal layer. The cavity is disposed contiguous to the region of reduced cross-sectional area and is dimensioned to promote vaporization of the metal at the region of reduced cross-sectional area to expose the colored polymer strip when a fault condition exists along the first conductive path.
In accordance with another aspect of the present invention, there is provided an electric fuse, comprised of a tubular casing formed of an electrically insulating material having a first conductive component attached to a first end of the casing and a second conductive component attached to a second end of the casing. A fusible element within the casing is electrically connected to the first and second conductive components. The first fusible element has a first resistance. An indicator is provided on the casing. The indicator is comprised of a first layer comprised of a non-flammable, dyed material and a second layer comprised of a conductive material deposited on the first layer. The second layer has an area of reduced thickness and a predetermined resistance greater than the resistance of the fusible element. The indicator is mounted to the casing with the second layer electrically connected to the first and second conductive elements in parallel with the fusible element and with the first layer of dyed material between the second layer and the casing. A third layer of a clear, polymeric material covering the indicator and at least a portion of the casing wherein the area of reduced thickness of the second layer is visible through the third layer. The second layer of conductive material is dimensioned to vaporize and expose a portion of the first layer when the fusible element experiences a fault condition that eliminates the electrical connection between the first and second conductive elements. The indicator provides a first visual indication when the fault condition results from a short circuit and a second visual indication when the fault condition results from an over current fault condition.
It is an object of the present invention to provide an electric fuse having an indicator for indicating whether the fuse has become non-conducting or has blown.
Another object of the present invention is to provide an electric fuse as described above that provides an indication whether a non-conducting, i.e., blown, fuse experienced an excessive, short circuit condition or a sustained overload condition.
A still further object of the present invention is to provide an indicator component as described above that is reliable and may be used in most types of electric fuses.
These and other objects will become apparent from the following description of a preferred embodiment of the invention taken together with the accompanying drawings and the appended claims.